This invention relates to a solid state imaging device and a manufacturing method thereof.
A known solid state imaging device has a pattern, as shown in FIG. 1, of photo-electro converting elements or element regions 1 and charge transfer elements or element regions 2.
On manufacturing, an insulation film 4 is formed on a semiconductor substrate 3 of the P conductivity type, as shown in FIG. 2A. A silicon nitride (SiN) film 5 of a predetermined pattern is formed on the insulation film 4. An impurity ion of the P conductivity type is implanted into the substrate 3, using the SiN film 5 as a mask, thereby forming p.sup.+ impurity regions 6. Thereafter, the structure is subjected to a local oxidation process in an oxide atmosphere of more than 1,000.degree. C., thereby expanding those regions 7 of the insulation film 4 not covered by the SiN film 5, as shown in FIG. 2B. The regions 7 are for the elements 1 and 2. Then, the SiN film 5 is removed, and an impurity ion of the N conductivity type is implanted into the substrate 3, using the insulation film 4 as a mask, thereby forming N impurity regions 8, as shown in FIG. 2C. The impurity regions 8 form photo-electro converting elements 1. The structure is then subjected to a thermal diffusion process so that the impurity regions 8 have the predetermined thickness in the substrate 3. An impurity ion of the P conductivity type is implanted into the substrate 3, thereby forming P impurity regions 9. The impurity regions 9 prevent a depletion layer at the surface of the substrate 3 from occurring, which, in turn, prevents a leak or dark current from flowing through the substrate surface.
However, with the conventional manufacturing method as described above, the separating regions 7 of the film 4 expand in the lateral and vertical directions of the substrate 3 at the local oxidation step. The lateral expansion of the separating regions 7 decreases the areas of the photo-electro converting element regions and charge transfer element regions. This lowers the conversion efficiency of the converting elements and the transfer efficiency of the transfer elements. In order to avoid the reduction of the converting regions and transfer regions, it is required that the size L1 of those regions of the SiN film 5 which correspond to the separating regions 7 be reduced. However, the size L1 of the regions of the SiN film 5 are required to have a value equal to or larger than a value which is critically defined from the aspect of the lithography technique. That is, from this aspect, the size L1 can not be considerably reduced. Therefore, in the actual manufacturing process, the size L2 of those regions of the SiN film 5 which correspond to the converting regions is largely set. However, this decreases the packing density of the elements, which, in turn, increases the chip size of the imaging device.
Also in the conventional manufacturing process, the impurity regions 6 and 9 are formed at separate steps to define step portions therebetween. These step portions are inconvenient for the high packing density of the elements of the imaging device.